MTD: nandsim: add option to use a file to cache pages

Add a new module parameter 'cache_file' which causes nandsim
to use that file instead of memory to cache nand data.
Using a file allows the simulation of NAND that is bigger
than the available memory.
Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>